For many years, it has been traditional practice for commercial tradesmen engaged in the installation of wall coverings, ceiling coverings, and the like, with overlapping edges (double lapped edges) to use a conventional straight edge and a cutting blade in cutting through the overlapping edges of adjacent covering sheets. It is difficult in using such a straight edge to achieve a straight cut along the length of the double lap because it is difficult for the tradesman to apply firm pressure against the straight edges at all times and hence the straight edge tends to wiggle or move out of place as the cutting blade is drawn along one side of the straight edge. This is particularly so if the blade is at the top or bottom end of the straight edge, where the leverage caused by the pressure of the blade against the straight edge is greater. As a consequence, it is not unusual to see wavey or uneven edges at the joints between adjacent sheets of wall covering such as wallpaper or vinyl backed wall coverings, which are commonly used in many interior wall covering or decorating applications.
Another problem with using an ordinary straight edge is that to make the job pay the installer must work rapidly and this tends to cause the installer to attempt to minimize the number of times the straight edge must be moved along the edge cut. In other words, he will endeavour to draw the cutting knife from the top to the bottom of the straight edge. Since the bending moment is much greater at the top and bottom regions of the straight edge, this practice leads to more frequent displacements of the top and bottom edges of the straight edge and thus uneven joint lines occur between adjacent wall covering sheets.